You can have as many AV utilities on your PC as you want so long as they aren't running in the background at the same time. If you have more than one running as a background/service you stand a very good chance of having a conflict between the programs (AV progs by nature require deep access to your OS and don't like it when other programs are attempting to use/control the same resources). Once you've got an AV conflict it can often be verrrryyyyy difficult to restore full functionality without a system restore/reinstall.